Our last Post discussed passive stabilisers.
This shot shows Laurie holding a bird, connected to a winch for lowering into the water. You can see the holes on top of the bird for adjusting the stabilising effect
Active stabilisers consist of a fin mounted below the waterline extending out each side of the hull using mostly hydraulics but in some systems electric motors to move them like aircraft ailerons to counteract rolling motion.
Envoy out of the water with stabilising fin visible
Envoy has hydraulically driven fins with the hydraulic pump powered by vee-belts from the main engine crankshaft and a 12 volt DC pump providing a constant flow of sea water to cool the hydraulic fluid. An electronic motion detector signals the hydraulics to move the fins to suit differing sea conditions with adjustment available via two thumb wheels in a pilot house control box.
The main advantages of this active system are that it’s more effective than passive stabilisers providing 80-90 per cent roll reduction (compared to about 70 per cent), it deploys quickly with no physical effort required, it’s easily adjustable using the electronic control system and causes negligible speed reduction.
However on the negative side the initial and ongoing cost of active stabilisers is expensive and during ten years we’ve spent several thousands of dollars replacing seals, sea water cooling pumps, vee-belts, filters, a hydraulic pump with its load adaptor, a servo valve and electronic components. Another issue is that they are highly specialised equipment and generally require trained and authorised engineers to provide service.
We’re just about to make another significant investment to remove the fins and replace the through-hull seals, something which is required about every five years. This is a job the majority of owners including ourselves wouldn’t attempt do themselves and special equipment is needed to remove the fins.
Set of through-hull seals
We’re also going to make a modification so that when the system is not in use the fins hydraulically lock in the centre position. Currently the fins move at rest (which is noisy and potentially damaging) in all but the most sheltered of anchorages, so when necessary we secure them in the centred position through a fiddly process of using manual locking bolts. The actions of locking them and then unlocking them takes about 15 minutes.
An active stabilisation system also requires a significant investment in spare parts and we carry a spare sea water pump, hydraulic motor, load adaptor, set of vee belts and oil filter. The fins can potentially be fouled by flotsam or lines although this has only happened to us once in a marina. Although some active stabilisation systems can be used at anchor with a generator running to provide power ours cannot, in any case we wouldn’t want to have an engine running overnight.
Partially hidden by the starboard side of the wheel is the stabilisers’ control system
Using both systems we’ve occasionally found it necessary to alter course by about 30 degrees to “tack” and take the waves more on Envoy’s bow when encountering closely-spaced, steep waves over about two and a half metres breaking directly on our beam.
A full displacement hull like Envoy definitely requires some form of stabilisation and the vast majority if not all Nordhavns have a system fitted. We’re pleased to have both active and passive systems aboard Envoy but if restricted to one would undoubtedly select the passive system for its reliability, economy and ability to use at anchor.
Next Post will take a light-hearted look a the use of the title “Captain”.